If humans had been present to build log structures during the Carboniferous period (though they were not!), which plant types would have been suitable sources of logs?

The plant types suitable during that period would be ferns, horsetails, and lycopods.

Explanation:

A geologic period and system, which covers 60 million years from the termination of the Devonian period to the start of the Permian period is known as the Carboniferous period. During the Carboniferous period, the plant life was luxuriant and extensive, mainly at the time of Pennsylvanian. It comprises ferns and fernlike trees, that is, the giant horsetails known as club mosses, calamites, or lycopods like Sigillaria and Lepidodendron.  

Thus, if humans had been present to construct log structures during the Carboniferous period, then they would have used logs of ferns, horsetails, and lycopods for construction work.  

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